Depends on the brand and type. Just like grout on tile, some want the cement to be fully cured, some don't care. You'd have to read the instructions on the specific stuff to be used. It could be anywhere from immediately to a week or more. Cement products are usually rated to be 'cured' in 28-days, and some don't want anything applied until that time, but there are some that can be applied (sometimes, way) before then.